Search for RR Lyrae Stars in DES Ultrafaint Systems: Grus I, Kim 2, Phoenix II, and Grus II

MNRAS 490, 2183–2199 (2019) doi:10.1093/mnras/stz2609 Advance Access publication 2019 September 24 Search for RR Lyrae stars in DES ultrafaint systems: Grus I, Kim 2, Phoenix II, and Grus II 1‹ 1 1† 1 C. E. Mart´ınez-Vazquez´ , A. K. Vivas , M. Gurevich, A. R. Walker, Downloaded from https://academic.oup.com/mnras/article-abstract/490/2/2183/5573280 by University College London user on 09 December 2019 M. McCarthy,2 A. B. Pace ,3 K. M. Stringer,3 B. Santiago,4,5 R. Hounsell,6 L. Macri,3 T. S. Li,7,8 K. Bechtol,9,10 A. H. Riley ,3 A. G. Kim,11 J. D. Simon,12 A. Drlica-Wagner,7,8 E. O. Nadler,13 J. L. Marshall,3 J. Annis,7 S. Avila ,14 E. Bertin,15,16 D. Brooks,17 E. Buckley-Geer,7 D. L. Burke,13,18 A. Carnero Rosell ,5,19 M. Carrasco Kind,20,21 L. N. da Costa,5,22 J. De Vicente,19 S. Desai,23 H. T. Diehl,7 P. Doel, 17 S. Everett,24 J. Frieman,7,8 J. Garc´ıa-Bellido,14 E. Gaztanaga,25,26 D. Gruen ,13,18,27 R. A. Gruendl,20,21 J. Gschwend,5,22 G. Gutierrez,7 D. L. Hollowood,24 K. Honscheid,28,29 D. J. James,30 K. Kuehn,31,32 N. Kuropatkin,7 M. A. G. Maia,5,22 F. Menanteau,20,21 C. J. Miller,33,34 R. Miquel,35,36 F. Paz-Chinchon,´ 20,21 A. A. Plazas ,37 E. Sanchez,19 V. Scarpine,7 S. Serrano,25,26 I. Sevilla-Noarbe,19 M. Smith,38 M. Soares-Santos ,39 F. Sobreira,5,40 M. E. C. Swanson,21 G. Tarle,34 and V. Vikram41 (DES Collaboration) Affiliations are listed at the end of the paper Accepted 2019 September 12. Received 2019 September 11; in original form 2019 July 4 ABSTRACT This work presents the first search for RR Lyrae stars (RRLs) in four of the ultrafaint systems imaged by the Dark Energy Survey using SOAR/Goodman and Blanco/DECam imagers. We have detected two RRLs in the field of Grus I, none in Kim 2, one in Phoenix II, and four in Grus II. With the detection of these stars, we accurately determine the distance moduli for these ultrafaint dwarf satellite galaxies; μ0 = 20.51 ± 0.10 mag (D = 127 ± 6 kpc) for Grus I and μ0 = 20.01 ± 0.10 mag (D = 100 ± 5 kpc) for Phoenix II. These measurements are larger than previous estimations by Koposov et al. and Bechtol et al., implying larger physical sizes; 5 per cent for Grus I and 33 per cent for Phoenix II. For Grus II, of the four RRLs detected, one is consistent with being a member of the galactic halo (D = 24 ± 1 kpc, μ0 = 16.86 ± 0.10 mag), another is at D = 55 ± 2 kpc (μ0 = 18.71 ± 0.10 mag), which we associate with Grus II, and the two remaining at D = 43 ± 2 kpc (μ0 = 18.17 ± 0.10 mag). Moreover, the appearance of a subtle red horizontal branch in the colour–magnitude diagram of Grus II at the same brightness level of the latter two RRLs, which are at the same distance and in the same region, suggests that a more metal-rich system may be located in front of Grus II. The most plausible scenario is the association of these stars with the Chenab/Orphan Stream. Finally, we performed a comprehensive and updated analysis of the number of RRLs in dwarf galaxies. This allows us to predict that the method of finding new ultrafaint dwarf galaxies using two or more clumped RRLs will work only for systems brighter than MV ∼−6 mag. Key words: stars: horizontal branch – stars: variables: RR Lyrae – galaxies: dwarf – galaxies: individual (Grus I, Kim 2, Phoenix II, Grus II). E-mail: [email protected] † Former research inter student. C 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society 2184 C. E. Mart´ınez-Vazquez´ et al. periods (∼0.2–0.45 d), lower amplitudes, and almost sinusoidal 1 INTRODUCTION light variations. RRLs are found in stellar systems that host an The Sloan Digital Sky Survey (SDSS; York et al. 2000) initiated the old (t > 10 Gyr) stellar population (Walker 1989; Catelan & era of large-area, deep, multicolour imaging sky surveys. One of Smith 2015). They are excellent standard candles due to their well- the results was the discovery of a new class of objects, ‘ultrafaint’ established period–luminosity relation (see e.g. Caceres´ & Catelan dwarf (UFD) galaxies, the first examples being Willman 1 and 2008; Marconi et al. 2015) that have been primarily calibrated Ursa Major I (Willman et al. 2005a,b). These UFDs extend the with field stars, first using Baade–Wesselink techniques (Fernley spectrum of properties of ‘classical’ Local Group dwarf galaxies to et al. 1998) and then trigonometric parallaxes from HST/Hipparcos Downloaded from https://academic.oup.com/mnras/article-abstract/490/2/2183/5573280 by University College London user on 09 December 2019 5 a lower mass regime (L<10 L; MV > −8 mag). Since these first (Benedict et al. 2011)orGaia (Muraveva et al. 2018). Therefore, discoveries, more than 50 UFDs have been found in the Milky Way the detection of at least one RRL in a UFD or star cluster provides (MW) neighbourhood (Simon 2019). UFDs appear to be possibly an accurate distance independent of other estimates, thus allowing the oldest and most primitive of galaxies (Bose, Deason & Frenk determination of absolute magnitude and physical size. In addition, 2018; Simon 2019). According to the hierarchical galaxy formation the presence of RRLs will confirm the existence of old stellar model (White & Frenk 1991), large galaxies are built up by the populations in these galaxies and their pulsation properties can also accretion of smaller galaxies; UFDs may be representative of the provide clues about the contribution of UFDs to the formation of basic building blocks of the galaxy formation process. If so, then the Halo of the MW (e.g. Fiorentino et al. 2015, 2017; Vivas et al. they are excellent probes to test the galaxy formation models and 2016). also to study the early Universe. In this paper, we focus our attention on four ultrafaint systems In the race to find new UFDs, the combination of the wide field imaged in the data collected by DES. From the farthest to the closest, of the Dark Energy Camera (DECam; Flaugher et al. 2015) with the they are Grus I, Kim 2, Phoenix II, and Grus II (Bechtol et al. 2015; large aperture of the CTIO Blanco 4m telescope (etendue´ = collect- Drlica-Wagner et al. 2015; Kim et al. 2015; Koposov et al. 2015). ing area × field of view = 38 m2 deg2), makes DECam + Blanco We obtain multiband (gri) and multiepoch photometry in order to the pre-eminent discovery machine in the Southern hemisphere. search for RRLs in these systems to better constrain their distances DECam observations, in particular those of the Dark Energy and satellite nature. Survey (DES; The Dark Energy Survey Collaboration 2005)and This paper is structured as follows. In Section 2, we present a MagLites (Drlica-Wagner et al. 2016) surveys, have contributed to summary of the observations. In Section 3, we explain the details of the discovery of more than 20 ultrafaint stellar systems undetectable the data reduction process. In Section 4, we describe the detection, in the past (e.g. Bechtol et al. 2015;Drlica-Wagneretal.2015; classification, and determination of the mean properties of the Kim&Jerjen2015; Kim et al. 2015; Koposov et al. 2015; Koposov discovered RRLs in the four ultrafaint satellite systems. In Section 5, et al. 2018; Luque et al. 2016, 2017; Martin et al. 2015; Martin et al. we discuss each galaxy individually and determine their distances. 2016a; Mau et al. 2019; Torrealba et al. 2018). The fact that many In Section 6, we show the correlation between the number of RRLs of them are close to the Magellanic Clouds suggests a possible and the total magnitude of the host galaxy and how this relation association (e.g. Jethwa, Erkal & Belokurov 2016; Erkal et al. behaves for galaxies fainter than MV –6 mag. Finally, in Section 7 2018; Fritz et al. 2019;Jerjenetal.2018; Kallivayalil et al. 2018). we present the conclusions of this work. This scenario of satellites of satellites is predicted by cosmological simulations at the time of infall (e.g. Sales et al. 2011; Deason et al. 2 OBSERVATIONS 2015; Wheeler et al. 2015; Pardy et al. 2019). Before the discovery of the UFDs, dwarf galaxies and globular 2.1 Targets clusters occupied well-defined locations in the MV versus half- light radius (rh) plane. However, for some of the new discoveries, Of the 17 ultrafaint systems published by Koposov et al. (2015), particularly the most compact ones with MV −4 mag (e.g. Bechtol et al. (2015; DES year 1), and Drlica-Wagner et al. (2015; Contenta et al. 2017), it is not clear whether they are star clusters or DES year 2), we decided to choose four of them (Grus I, Kim 2, UFD galaxies (see fig. 5 in Drlica-Wagner et al. 2015; Conn et al. Phoenix II, and Grus II) based on their visibility during the A- 2018a,b). Because they are low-mass systems, the scarcity of stars semester, which is when the observing time was granted. We and the large contamination by field stars make the determination also took into account their extension in the sky, so that they of their morphological parameters a challenge.

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